Apparently it's part of a group who call themselves the UX, an underground
organization that improves the hidden corners of Paris, including those the French
government has ignored or lacks funds for.

The group includes architects and historians, and in 2007 they received attention for
restoring the famous clock in the Panthéon, IN SECRET. Never caught, they
announced their work upon completion at a meeting with the DJ 4DM1Nistrator of the
Panthéon, who called the police.

there was an issue of National Geographic about it a while back. I thought the group was awesome, but a few of them seemed like radicals. What I mean is some of them didn't care about their surroundings, like delinquents.